finagle: rename FailedFast onOrOff to enabled

Problem

`onOrOff` is a confusing name for a boolean parameter.

Solution

Rename it to `enabled`.

RB_ID=720781
TBR=true
USER_HOOK_ARGS=--i-am-evil -x macaw-swift/.hooks/PRESUBMIT/check-swoop

CHANGES

@@ -18,6 +18,8 @@ New Features
 Runtime Behavior Changes
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
+  * finagle-core: FailFast parameter renamed from `onOrOff` to `enabled`. ``RB_ID=720781``
+
   * finagle-core: When evaluating NameTree unions, return components of the union in Ok state rather
     than waiting for all components to be Ok. This enables resilience of unions when part of the
     tree cannot be resolved. ``RB_ID=697114``

finagle-core/src/main/scala/com/twitter/finagle/builder/ClientBuilder.scala

@@ -772,17 +772,17 @@ class ClientBuilder[Req, Rep, HasCluster, HasCodec, HasHostConnectionLimit] priv
     "No longer experimental: Use failFast()." +
     "The new default value is true, so replace .expFailFast(true) with nothing at all",
     "5.3.10")
-  def expFailFast(onOrOff: Boolean): This =
-    failFast(onOrOff)
+  def expFailFast(enabled: Boolean): This =
+    failFast(enabled)
 
   /**
    * Marks a host dead on connection failure. The host remains dead
    * until we successfully connect. Intermediate connection attempts
    * *are* respected, but host availability is turned off during the
    * reconnection period.
    */
-  def failFast(onOrOff: Boolean): This =
-    configured(FailFast(onOrOff))
+  def failFast(enabled: Boolean): This =
+    configured(FailFast(enabled))
 
   /**
    * When true, the client is daemonized. As with java threads, a

finagle-core/src/main/scala/com/twitter/finagle/service/FailFastFactory.scala

@@ -34,12 +34,12 @@ object FailFastFactory {
    * For details on usage see the
    * [[https://twitter.github.io/finagle/guide/FAQ.html#why-do-clients-see-com-twitter-finagle-failedfastexception-s FAQ]]
    */
-  case class FailFast(onOrOff: Boolean) {
+  case class FailFast(enabled: Boolean) {
     def mk(): (FailFast, Stack.Param[FailFast]) =
       (this, FailFast.param)
   }
   object FailFast {
-    implicit val param = Stack.Param(FailFast(onOrOff = true))
+    implicit val param = Stack.Param(FailFast(enabled = true))
   }
 
   /**